Devices for measuring lung functions include, for example, spirometers or pneumotachographs, by means of which lung volume or a respiratory characteristic of a test subject can be determined over a period of time. A time curve of the breathing pressure and breathing volume is of particular interest for diagnosing existing or incipient respiratory illnesses or other pathologies in the context of the respiratory system of a test subject. Determining a volume flow over time of the air inhaled or exhaled by a test subject is particularly desirable.
Directly determining the volume flow of respiration in the conventional manner, however, is difficult, and requires corresponding sensors that are typically prone to failure or contamination. For example, a sensor having a turbine wheel that can directly measure a volume flow can be used for measuring volume flow in a spirometer. Such sensors, however, are generally relatively expensive, and must also be located directly in the flow of the breathing air of a patient. They are thereby unavoidably exposed to contamination by saliva or potentially by pathogens, so that extensive cleaning measures and regular functional checks are necessary.
Conventional windmill-type spirometers and pneumotachographs based on volume flow sensors also require extensive calibration to guarantee the accuracy of measurements.